The term "acoustic near field" refers to the area of a structure where both radiating and non-radiating (i.e., evanescent) acoustic waves exist. The term "acoustic far field" refers to the area around a structure where only the radiating acoustic waves exist and the non-radiating acoustic waves have decayed. For a given structure producing radiating and non-radiating waves, the radiating waves produce observable responses in the acoustic far field while the non-radiating acoustic waves do not. In mapping the acoustic near field of the structure, both the radiating and non-radiating acoustic waves are measured. Using these measurements and signal processing techniques, information on the types of structural waves traveling along the structure and their contribution to the observable responses in the acoustic far field can be obtained.
To map the acoustic near field of a structure, three measurement techniques have been employed in the prior art. The structure can be moved with respect to a scanning system, the scanning system can be moved with respect to the structure, or the scanning system can include a multitude of measurement devices surrounding the structure. In the first two techniques, the scanning system is a one-dimensional array of measurement devices and either the structure is moved or the array of measurement devices is moved. However, both of these techniques require intricate manipulation and physical coordination between the structure and the scanning system. With the third technique, the scanning system consists of measurement devices located at all positions where measurements of the acoustic near field are desired. The advantage of this technique is that neither the structure nor the measurement devices need be moved. However, this technique is often cost prohibitive as numerous measurement devices are required.